Self hardening ASTM Class C fly ash, hereafter referred to as "fly ash", results primarily from the burning of pulverized coal mined from the Powder River basin in Wyoming. Fly ash contains various compounds including calcium, silicon, aluminum and iron oxides, and tends to rapidly gel and harden when mixed with water. The self-hardening characteristic makes fly ash useful for solidifying sludge material including non-hazardous or hazardous wastes. Fly ash is also useful when mixed with road base materials to stabilize and solidify road base materials in preparation for surface treatment and placement of the finish surface material (bituminus asphalt, etc.). As used herein, "solidifying agent" means a flowable fly ash which can be hardened upon the addition of water with or without the addition of mineral fillers such as earth and stone aggregates.
One major problem with the use of fly ash is that it is difficult to transport, except in totally enclosed tank type cars or trailers, without producing substantial quantities of airborne dust. The small size and spherical shape of the fly ash particles and its relatively low density make it especially prone to extreme dust problems. One way of preventing dust production is to add water to the fly ash. However, if an excessive amount of water is added, the fly ash will solidify making it hard to dispose of and practically useless as a solidifying agent for future combination with waste or road bed materials.
Most of the prior wet methods for controlling fly ash dust require a water filled pond. Variations include (1) underwater discharge from pressurized pneumatic tanker trucks, (2) slurry discharge from the power plant into an ash pond, and (3) discharge from pneumatic tankers through slurry mixers at the ash pond. There are several problems with these methods. First, once the fly ash is immersed in the pond it is useless for future mixing with waste or road base materials. Second, these methods yield a very low disposed ash density, meaning that the unit cost for pond volume will be significantly higher than for dry disposal methods. Third, use of a water filled pond creates the potential for hazardous subterranean leaching problems and utilizes high volumes of water.
Other wet methods have been tried. For example, one approach involves dry hauling and dumping followed by wetting with a water truck and mixing with a dozer blade. A principal problem with this method is that dozer blade mixing is messy and imprecise. Another wet method involves wetting with conventional ash conditioners at the silos followed by hauling of the moistened material to the landfill. This method requires starting and stopping a silo based conditioner each time a truck is loaded. Starting and stopping the conditioner causes variations in the water to ash ratio leading to inconsistent product treatment and equipment blockage. The material is very difficult to unload from trucks because it tends to stick and harden in the trucks.
Therefore, due to these and other problems which are unsolved by the prior art, an object of the present invention is to provide a substantially dust-free fly ash composition which is amenable to being easily transported for efficient disposal or use as a solidifying agent with waste or building materials.
It is also an object to provide a method for processing fly ash which substantially prevents dust production without causing the fly ash to fully harden, thus maintaining the fly ash' capacity to act as a solidifying agent.
Another object is to develop an apparatus for carrying out the new process. It is desireable that the apparatus be mobile so that the process can be carried out at various sites which may be remote from the coal burning station.